Conventionally, as an internal combustion engine system of this kind, there is proposed the one that determines misfire of an engine based on a torque correction amount of a motor at the time of vibration control for canceling off a torque fluctuation of the engine by the motor in a vehicle mounted with the motor that is capable of generating electric power and connected to the crankshaft of the engine (for example, see Patent Document 1). In this system, a misfire is determined based on the rotation fluctuation at a crank angle position when vibration control by the motor is not carried out, and when the engine is operated at high rotation with high torque even though the vibration control by the motor is carried out, and misfire of the engine is determined based on the torque correction amount of the motor at the time of vibration control when the engine is operated at low rotation or operated with low torque while the vibration control by the motor is carried out.
Patent Document 1: Japanese Patent Laid-Open No. 2001-65402